Lithium-based battery cells are an attractive energy source for portable applications due in part to their ability to provide relatively high energies and long cycle life. Lithium is the lightest of all the metals, with a high electrochemical potential, thus providing high energy densities. Rechargeable batteries using lithium as the electrochemical material are capable of providing higher energy to weight ratios than those using other chemistries.
The voltage of lithium ion batteries corresponds to the total energy capacity and depends on the choice of the anode and cathode couples used in the battery cell. High voltage cathode materials include, among others, LixMnOy, LixCoOy, and LixNiOy. Of these, the nickel based material, particularly LiNiO2, has the highest capacity, and as a result has become a focus area for the enhancement of lithium cell energy. While cells incorporating this material yield higher energy, assessing all the energy leads to material instability that in turn results in poor cycle life characteristics of the cells.
The most widely used active material for positive electrodes for lithium ion secondary batteries is lithium-cobalt composite oxide. However, lithium-cobalt composite oxide electrodes require an expensive cobalt compound for raw material, increasing the cost of the positive electrode material, and resulting in the higher cost of the secondary battery. Thus, there is a need for an inexpensive substitute for the active material for positive electrodes.
Research is currently underway on lithium-metal composite oxides, where the metal is selected from manganese and nickel, as a replacement for the lithium-cobalt composite oxide. In particular, the lithium-nickel composite oxide manifests the same high voltage as the lithium-cobalt composite oxide, and has a theoretical capacity greater than that of the lithium-cobalt composite oxide, and the Ni raw material is less expensive than Co, and available in stable supply. In addition, lithium-nickel composite oxide electrodes have a higher charge capacity and discharge capacity, and improved cycle characteristics in comparison to lithium-cobalt composite oxide electrodes. However, lithium-nickel composite oxide electrodes have the following disadvantages: (i) discharge capacity is less in comparison to charge capacity; (ii) the irreversible capacity, or so called “retention” defined by the difference between the charge capacity and discharge capacity is considerable; and (iii) battery performance is comparatively easily degraded when used in high or low temperature environments.
To improve cycle characteristics, a different element (e.g., B, Al, In and Sn, or Li(Ni, Co)O2 composite oxide) may be added for substitution in the lithium-nickel composite oxide. While this improves cycle characteristics, it also narrows the range within which intercalation-deintercalation of the lithium ions is obtained, and tends to reduce discharge capacity. This reduction in discharge capacity is known to be particularly apparent under conditions of high load at high discharge currents, and conditions of high efficiency discharge at low temperatures where lithium ion mobility in electrolyte is reduced at low temperatures.
Output characteristics of secondary batteries at low and/or high temperatures are important when the secondary batteries are installed in equipment used in environments in which temperature variation is large. In particular, if use in cold regions is considered, the secondary batteries must have sufficient output characteristics required at low temperature. Improvement in the output characteristics at low temperature is therefore an important consideration when the lithium ion secondary batteries with lithium-nickel composite oxide are installed in e.g., motor vehicles.
Carbon-based materials such as crystalline graphite with high crystallinity are used for negative electrodes in lithium-ion batteries. This type of graphite has a layered structure, and lithium ions are intercalated from the edge of the layered graphite to the intervals of graphite layers during charging of a secondary battery, thereby producing a Li-graphite intercalation compound.
When graphite is used as a negative active material to prepare a negative electrode, the planes of the graphite layers are parallel to the plane of the collector, since most graphite is flake-shaped. Therefore, the edges of the graphite layers are aligned in a direction perpendicular to the positive electrode and, therefore, lithium ions which are deintercalated from the positive electrode cannot easily intercalate to the graphite layers during charging. In particular, when a battery is charged at a high rate, lithium ions are insufficiently intercalated to the graphite layers and discharge characteristics consequently deteriorate.
In addition, since a lithium secondary battery is generally charged under constant current and constant voltage (CC-CV) and it is discharged under constant current, lithium ions that are deeply intercalated to the crystalline graphite layers are not fully deintercalated when the battery is discharged at high rates, thereby deteriorating cycle life characteristics. Cycle life characteristics of a conventional lithium secondary battery further deteriorate because the lithium ions that deintercalate are insufficient to intercalate to the graphite layers, and too many lithium ions remain in the graphite.
Electrical resistivity of a graphite-containing composition in the inner plane direction of a graphite layer (an (ab) plane or a (002) plane) is about 1000 times that of the plane direction of the graphite layer. Therefore, if the alignment of graphite can be controlled, anisotropy of graphite may decrease or it may be eliminated, and the graphite can be used in electronic appliances as well as in batteries. However, for a lithium ion battery with a carbonous (graphite) anode, the theoretical amount of lithium which can be intercalated by the anode is only an amount of ⅙ per carbon atom at the most. Thus, when the amount of lithium intercalated by the anode is made to be greater than the theoretical amount upon charging or when charging is performed under condition of high electric current density, there will be an unavoidable problem of lithium deposition in a dendritic state (that is, in the form of a dendrite) on the surface of the anode. This will result in causing internal-shorts between the anode and the cathode upon repeating the charge-and-discharge cycle. Therefore, it is difficult for the lithium ion battery with the carbonous (i.e., graphite) anode to achieve a high capacity.
Rechargeable lithium batteries in which a metallic lithium is used as the anode have been proposed. Metallic lithium electrodes exhibit a high energy density. However, the charge-and-discharge cycle life is extremely short, because the metallic lithium reacts with impurities such as moisture and organic solvents present in the electrolyte solution and form an insulating film. In addition, the metallic lithium anode has an irregular surface with portions to which electric field is converged. These factors lead to generating a dendrite of lithium upon repeating the charge-and-discharge cycle, resulting in internal-shorts between the anode and cathode. As a result, the charge-and-discharge cycle life of the rechargeable battery is extremely shortened. A lithium-aluminum alloy has been used in an attempt to eliminate the problems of the metallic lithium. However, because the lithium alloy is hard it is difficult to produce a spiral-wound cylindrical rechargeable battery.
Thus, there is a need for new materials for electrodes that can improve electrode performance and, in particular, the charge-and-discharge cycle of lithium ion batteries. The present invention is directed to overcoming these and other limitations in the art.